building my own kitchen cabinet doors...

...and I've got a couple of questions. I've just about finished building the cabinets and face frames and I'm just starting to think seriously about the doors. The cabinets and face frames are birch, with semi-gloss (water-based) polyurethane. They look OK, but kinda boring (I wanted to use the oil-based poly, but the wife thought the water-based was nicer). We've decided to go with Shaker style cabinet doors, and I've got the router bits I need, so I'm about ready to get started on that.

Anyways, I was wondering if anybody has any thoughts about using a different wood species for the doors than the cabinets/face frames? I think walnut would look great, especially in contrast to the rather bland birch. However, I don't think I've ever seen anything like this before. And how about if I were to get really wild and make the door stiles and rails, say, walnut, but the interior (flat panel) birch plywood---or maybe even maple for the panels. OK, even I think that would probably be too much, but what about doors that contrast with the cabinets? Any thoughts? Thanks.